His departure comes on the heels of JWT's appointment in June of DDB Chief Creative Officer Matt Eastwood as global chief creative officer, a post that had been vacant since 2009.

It also follows the recent departure of top New York office creative leaders. Co-Chief Creative Officers Ryan Kutscher, whom Mr. Benjamin hired from Crispin, and Matt MacDonald, promoted by Mr. Benjamin, left separately last year. The agency has not named successors, instead putting creative leadership in the hands of existing and newly appointed executive creative directors.

Mr. Benjamin said his new venture was inspired by new opportunities in advertising and marketing. "Just the other day I was riding a Citibike, thinking this is the greatest outdoor advertisement ever," he said. "I've also been lucky to work on Macy's, Amex's Small Business Saturday. They do things ads are supposed to do, but they're not ads. I've been obsessed with ideas like that and for me, it's going to be fun creating a place where clients come to you and that's the expectation. Increasingly brands are looking wider than just ad agencies for creativity."

He's calling the business, for now, a "creative venture" because "I feel like every time someone starts something, they don't want to call it an ad agency, but then it ends up being an ad agency," he said. "I think it would be silly for me to say it's not an agency, but I do think in order for a startup to succeed, it's gotta be something different."

Mr. Benjamin's arrival at JWT ended a two-year vacancy in the North American leadership post after Ty Montague's exit, and he brought with him a pedigree of notable digital work on brands such as Domino's, Best Buy, Coke Zero and Kraft from his Crispin days. He went on to oversee JWT campaigns for Macy's, including the campaign that turned the award-winning "Yes, Virginia" animated special into a musical that kids could replicate in their own schools. He also worked on the Band-Aid Magic Vision app, which turned bandages into triggers for augmented-reality content starring Kermit the Frog, Miss Piggy and other Muppets.

The North America Chief Creative Officer departure leaves an obvious hole in the leadership of JWT's flagship office, but Mr. Eastwood said for now, he'll be working closely with the executive creative directors to oversee creative. They include former Marcel, Paris creatives Emmanuel Lalleve and Florent Imbert, who worked on Cartier's visually arresting 165th anniversary film "Odyssee de Cartier"; Andy Carrigan and Jon Zast, a duo from Anomaly who worked on Captain Morgan and Budweiser; and former Razorfish Executive Creative Director Frederic Bonn as an executive creative director on digital.

"I'm going to step in and look after New York as we try to recruit someone," Mr. Eastwood said."I'm very aware there's been a bit of a moving guard in terms of who's in charge, and I definitely want to find someone for the role."

Ultimately, Mr. Eastwood is looking for a new CCO who can oversee both North American duties, which largely consists of leading the North America creative council, and who can make New York a creative leader among the American shops. "You have a company with a brand new CEO [Gustavo Martinez, hired as president to success Bob Jeffrey at year's end], a brand new CCO and a flagship looking for great creative leadership," he said. "To me, it's a great job for someone looking to make a significant difference to the business."

As for his priorities for New York, Mr. Eastwood said, "For me creatively it has to be one of the strongest offices in the network. I've always believed when you're strong creatively in New York, you're strong across America and you have to be strong in America to compete globally."

Mr. Benjamin plans to open his new business, yet to be named, in about six months.